Foamed plastic sheet

ABSTRACT

An unmixed plastic sheet, which is made of a thermoplastic material and which is composed of a number of body segments that are adjacent to one another in a plane, interconnected and made of a closed-cell foam material. The body segments are thermoplastically welded to one another at their abutting lateral faces while forming flat weld seams. The flat weld seams form a plastic intermediate layer, which has few pores, and which is provided in the form of a web structure that, when viewed from above, is reticular and reinforces the plastic sheet. The inventive plastic sheet is preferably used as a core layer in sandwich composites.

The invention relates to a large area structural element containing aplurality of body segments arranged adjacent to one another in a plane,interconnected and made of a closed-cell foamed plastic, and a processfor manufacturing such a large area structural element and the usethereof.

It is known to employ a foamed thermoplastic as the core layer insandwich-type composites. The production of foamed plastic panels may beachieved e.g. by an extrusion process. The structural load-bearingcapacity, in particular the compressive strength of such core layersmade using extrusion processes is however limited. For demandingapplications, therefore, sandwich composites with the above mentionedcore layers are often not suitable as structural components.

However, for demanding applications such as e.g. structural componentsin transportation it is necessary to have sandwich composites thatexhibit a high degree of strength, in particular compressive strengthand stiffness, and core materials that exhibit high shear strength andstiffness. In order to achieve these properties, use is made e.g. ofstronger and much thicker outer layers. As a rule this leads to anundesirable increase in the specific weight of the sandwich composites.Further, the compressive strength of such sandwich composites can not beincreased in an unlimited manner by use of thicker outer layers.

It is therefore desirable not only for the outer layers but also thecore layers to exhibit greater compressive and shear strength, however,without having to forego the advantage of smaller thickness of foamedbodies.

This would enable sandwich composites with improved strength andstiffness characteristics to be achieved without significant increase inthe specific weight. On the other hand, the use of core layers withincreased stiffness and strength would also permit the use of thinnerouter layers.

Sandwich composites are therefore known that exhibit improved strengthby use of specific design of the core layer.

Described in DE 197 15 529 C1 for example is the production of asandwich composite with a core layer and outer layers on both sidesthereof, whereby the core layer is made up of horizontally abuttingpolygonals e.g. cube-shaped foam segments. In that case the individualfoam segments are covered over with a fibre layer and fitted together ina device such that the sides of the segments are in contact with thefibre layer in a strut-Ike manner. The strut-like fibre layers aresoaked with an impregnating medium. By hardening the impregnating mediumfibre-composite layers are produced between the foam segments, thusproviding a stiffening and strengthening effect on the core layer.

The described manufacturing process is, however, found to be verycomplicated and costly. Further, a core layer produced in accordancewith the described process is of a mixed type as the foam segments,fibre layers and impregnating medium result in at least three differentmaterials being used.

The document U.S. Pat. No. 4,837,060 describes a composite element witha core layer of mutually adhesively bonded or sealed cylindrical shapedfoam elements with outer layers provided on both sides of the corelayer. The axes of the cylinders of the foam elements run parallel tothe planes of the outer surface layers.

The document U.S. Pat. No. 4,536,427 describes a composite element witha core layer of mutually adhesively bonded balsa wood elements. Theadhesive bond is flexible and elastic in design with the result that thecore layer is able to bend.

The object of the present invention is to propose a large area, inparticular sheet-like structural element which is suitable for corelayers in sandwich-type composites, whereby the structural elementshould contain a foam with a closed-cell structure. A further object isa cost favourable process for manu-facturing the mentioned large areastructural element. The large area structural element should inparticular exhibit improved compressive strength in com-parison withconventional foam panels. The large area structural element should alsobe as unmixed in character as possible.

That objective is achieved by way of the invention in that thestructural element is made wholly of plastic, and the body segments arewelded together at their abutting lateral faces forming flat weld seams,whereby the flat weld seams form a plastic intermediate layer which hasfew pores or is pore-free and which is provided in the form of a webstructure that, when viewed from above, is reticular and reinforces theplastic sheet.

The large area structural element is preferably sheet-shaped. The largearea structural element is preferably in the form of a sheet-typeelement, in particular a block-shaped sheet element.

The large area structural element comprises a thermoplastic, preferablya poly-styrene (PS), acrylnitrile/butadiene/styrene-grafted coploymer(ABS). poly-ethylene (PE), polypropylene (PP), polyvinylchloride (PVC),polycarbonate (PC) and in particular polyethylene-terephthalate (PET),poly-phenylene-ether (PPE) or poly-blends thereof such aspoly(phenylene-ether)-polystyrene (PPE-FPS), or a styrene/acrylnitrilecopolymer (SAN).

The structural element according to the invention is preferably unmixedi.e. it is comprised of only one single plastic. The unmixed characterrequires in particular that the structural element contains no otherkind of adhesive for joining the body segments together.

The specific weight of a structural element according to the inventionis e.g. more than 20 kg/m³, preferably more than 40 kg/m³, in particularmore than 50 kg/m³ and e.g. less than 200 kg/m³, preferably less than130 kg/m³. The pore size of the foam is e.g. in the range 100 to 1000μm.

The weld seams are preferably formed by plastic of the body segmentwhich is melted then solidified after fitting the segments together. Tothat end the body segments exhibit, in particular at the abutting sides,zones in the form of areas that melt and re-solidify.

The thickness of the weld seams or melt zones is chosen such that thenetwork-like strut structure of the weld seams increases the compressivestrength of structural element with respect to area pressure. This meansthat the weld seams represent not only joins between two body segments,but at the same time stiffening or reinforcing struts of foam betweenthe body segments. The weld seams also effect stiffening of thestructural element with respect to shear and bending stresses. Theexpression of strength or thickness of the weld seams refers thereforenot only to a stable weld seam but also to effective stiffening orreinforcing of the structure.

The thickness of the melt/re-solidified zone at the side wall faces thatconstitute the weld seams is selected therefore such that the structuralelement exhibits in particular high compressive strength with respect tosurface pressure.

The body segments are preferably fitted and welded together in acontinuous manner i.e. free of voids. The body segments thereforepreferably exhibit a cross-sectional shape that enables the segments tobe fitted together in rows without gaps.

In a preferred version of the invention the body segments exhibit—in aplan view of the structural element—a poloygonal shape, in particular aneight, six, four or three sided shape. The body segments may in planview of the structural element exhibit e.g. a quadratic, rectangular,hexagonal or triangular outline which runs around the so called topfaces of the body segments.

The size of the body segments may vary according to the degree ofstiffness or compressive strength desired.

As each of the above mentioned geometries and sizes of body segmentsresults in large area structural elements with different properties, thegeometry and size of the body segments are determined primarily by thespecific require-ments placed on the structural element.

The body segments are especially preferably in the form of cubes orblocks. Body segments of block or cube shape may e.g. be arranged inseveral rows and columns, whereby the weld joints form a network-typestructure with intersecting longitudinal and transverse weld seams. Thesaid body segments may also be arranged in several rows that are offsetwith respect to each other, whereby the weld joints form continuoustransverse weld seams and offset longitudinal weld seams creating apattern like a brick wall.

Quadratic shaped body segments may e.g. (as viewing the structuralelement in plan view) exhibit a quadratic or rectangular shaped outlinewith side lengths (x, y) of 20 to 600 mm, preferably 30 to 400 mm, inparticular 40 to 100 mm (see also FIG. 1). A rectangular body segmentmay e.g. (as viewing the structural element in plan view) exhibit alength of 100 to 250 mm and a breadth of 30 to 50 mm.

The body segments may—viewing the structural element in plan view—alsoexhibit a curved outline shape e.g. concave or convex in whole or inpart. The body segments may also be shaped in a manner of interlockingbricks i.e. the body segments are shaped such that the individual bodysegments lock into each other. The body segments of the structuralelement according to the invention are also preferably congruent withrespect to each other.

In sheet-shaped structural elements the side faces of the body segmentsare also preferably perpendicular to the outer faces of the structuralelement.

The invention also relates to a process for manufacturing a large areastructural element containing a plurality of body elements of a foamedplastic that are aligned next to each other in a plane and joinedtogether.

The process is characterised by way of the following steps:

-   a) manufacture of closed-cell rod-shaped or column-shaped foamed    plastic bodies;-   b) welding together the long sides of the rod-shaped or    column-shaped foamed bodies into a plastic block thus creating weld    seams over that face, whereby the weld seams are then present as    low-pore or pore-free intermediate plastic layers;-   c) dividing the block of foamed plastic into individual large area    structural elements, in particular foam sheets, running transverse    or perpendicular to the longitudinal direction of the rod-shaped    foam bodies,    whereby the weld seams, as viewed in plan view of the structural    element, form a network like structure of struts.

The rod-shaped or column-shaped foam bodies are preferably manufacturedusing an extrusion process. The orientation of the material of the foambodies or body segments preferably is preferably stretched in thedirection of extrusion. Thereby, in particular polymer chains exhibitstretching in the direction of extrusion as a result of the extrusionprocess. The stretching of the material effects an improvement in themechanical properties, in particular the compressive strength in thedirection of stretching.

The foam bodies or body segments also preferably exhibit a cellstructure or cell arrangement that is oriented in the direction ofextrusion. The oriented cell structures of the foam body contribute toan increase in the compressive strength of the structural element.

In another version of the invention the rod-shaped or column-shaped foambodies may be taken from a prefabricated block of foam i.e. by cuttingor sawing. The said block of foam is manufactured by means of anextrusion process.

According to a specific version of the invention the production of thefoam bodies takes place via an extrusion process, whereby the foam bodycomprises a plurality of expanded polymer strands that are broughttogether. Drawing the strands together takes place immediately afterleaving the extrusion die by expanding the individual strands. Thereby,neighbouring strands make contact with each other, grow together orstick together forming a foam body. The foam body is then in the form ofa closed package of strands.

The strands lie preferably essentially parallel to each other and arearranged in the longitudinal or extrusion direction of the foam body.The manufacturing pro-cess may be arranged such that the individualstrands remain visible in the foam body or are brought together ormelted to give one single structure in which the individual strands areonly slightly or no longer detectable. The strands are preferably packedtogether so tightly that the individual strands come to rest fully incontact with each other without forming any spaces. The strands may,however, also be arranged or packed together such that channel-likespaces are formed between the individual strands.

The strands are usefully created via an extrusion tool e.g. die plate,whereby the extrusion tool contains a plurality of neighbouring openingsthrough which the polymer is extruded in the form of strands. Incross-section the said openings may be round e.g. circular or ellipticalshaped, or polygonal e.g. rectangular, quadratic or hexagonal. Theopenings may also be in the form of slits. The foamed strands may have adiameter of e.g. 3 to 50 mm, in particular 4 to 20 mm.

Situated downstream of the extrusion die there may also be anothershaping tool that specifies the outer contour of the foam body, in whichtool the strands are packed together on emerging from the extrusion die,so that the foam body takes on the cross-sectional shape of the shapingtool.

With the above manufacturing process it is possible to producerod-shaped or column-shaped foam bodies which have already been extrudedwith the desired cross-sectional shape and size so that the foam bodiescan be cut to length individually or already in combination with furtherrod-shaped or column-shaped foam bodies into body segments or containlarge area structural elements containing body segments.

Further, using the above manufacturing process it is possible to producefoam bodies of a cross-sectional size which is larger the requiredcross-section of rod-shaped or column-shaped foam bodies, so that in thesubsequent process-ing steps the extruded foam body is cut intoindividual rod-shaped or column-shaped foam bodies, whereby the strandsrun essentially in the longitudinal direction of the cut rod-shaped orcolumn-shaped foam bodies.

In a preferred version of the foam bodies or body segments manufacturedaccording to the above described process the orientation of theindividual strands of the body segments lies essentially perpendicularto the outer faces of the large area structural element made up of bodysegments. Thereby, as a result of the orientation of the strands, thecompressive strength is preferably greater in the longitudinal directionof the strands i.e. in the direction of extrusion than in the otherdirections.

The foam bodies may be produced using physical or chemical foamingmeans. In a preferred version the foam bodies are foamed physicallyusing CO₂. The introduction of the foaming agent may take place directlyin the extrusion device.

In a preferred version of the invention the extruded foam bodies arewelded into plastic blocks on their long sides i.e. along the long sidesthat come into contact with each other. Large area, in particularsheet-shaped structural elements are subsequently cut from these plasticblocks transverse or perpendicular to the long sides of the foam bodies.

The cutting of the large area structural elements out of the plasticblocks may be performed by sawing or using a thermal cutting process.

Instead of the above described process steps the foam bodies may also bealready cut into body segments whereby the individual body segments aresubsequently welded together to form large area structural elements.

The welding takes place preferably by melting the side faces of the bodysegments that are to be joined, followed by fitting these together andsolidifying the molten zones.

In a preferred version means for controlling the melting process duringwelding are employed that allow weld seams of a specific thickness orthickness range to be produced, whereby the thickness range is selectedsuch that the web-like strut structure of weld seams produces astiffening effect on the plastic sheet.

The welding process is usefully one of thermoplastic welding. Theproduction of the weld joint may take place by means of radiationwelding or contact welding. Plastic welding processes that can beemployed are e.g. welding with heating elements or infrared welding. Thewelding process may take place with or without weld-feed materials.

Compared to conventional foamed plastic sheets, the large areastructural elements according to the invention exhibit greater stiffnessand in particular greater compressive strength. The properties are dueessentially to the weld seams between the individual body segments. Theweld seams form a network of strut-like connections like a framework,whereby the weld seams are in the form of low-pore or pore-free, denseintermediate plastic layers.

The framework of strut-like connected weld seams increases thecompressive strength as the struts forming the intermediate plasticlayer are significantly less compressible than the foam body itself. If,for example, surface pressure is applied to the structural elementaccording to the invention (e.g. in the case of a sandwich composite viathe outer layers onto the structural element serving as core layer),then the compressive forces act mainly on the stiff strut-like structureand not on the foam bodies themselves.

The increase in stiffness of the structural element according to theinvention is also a result of the framework-like nature of the weldseams which lead to an increase in the torsional and bend strength ofthe structural element.

The structural element according to the invention finds applicationpreferably in composite elements, in particular composite sheets with atleast one outer sheet on one side of the structural element.

The structural element according to the invention finds highlypreferable application as core in sandwich composite elements, inparticular sandwich composite elements with outer layers on both sidesof the core layer. The sandwich composite elements may e.g. comprise alarge area structural element according to the invention with an outerlayer on both sides of the structural element. Further, it is alsopossible for there to be several layers on one or both sides of thestructural element. If the structural element according to the inventionis employed as a core layer in a sandwich composite sheet, thestructural element is usefully a sheet element.

The said layers or outer layers may be joined to each other and/or tothe core layer e.g. by adhesive joining means.

The outer layers may e.g. be rigid or flexible sheets of plastic orfibre reinforced plastic such as glass fibre reinforced plastic.Further, the outer layers may also be sheets of metal, in particularaluminium or an aluminium alloy. The outer layers are, in comparisonwith the core layer, usefully relatively thin sheets.

In spite of their low weight, sandwich composite elements withstructural elements according to the invention exhibit a high degree ofstiffness and excellent shear and compressive strength. Such sandwichstructures are therefore suitable especially for applications whichrequire components which are light in eight but structurally high loadbearing.

Sandwich-type composite elements with structural elements according tothe invention as core layer may e.g. be employed as constructionelements in the building industry.

Examples of such construction elements are walls, floors, ceilings,doors, dividing walls or cladding elements.

Sandwich composite elements with structural elements according to theinvention as core layer also find preferred application intransportation on land (e.g. road or railway vehicle manufacture), onwater (e.g. ship and boat building, equipment for water sports) or inthe air (e.g. aircraft construction). This may be e.g. sandwichcomposite elements according to the invention for trunk structures,loading platforms, walls, ceilings, floors, lids, cladding or partsthereof, on or as part of lorries or railway trucks for goods transport,or walls, ceilings, floors, intermediate walls, cladding elements,doors, lids or parts hereof, on vehicles for public transport such asomnibusses, trams, railway carriages or on ships such as passengerships, ferries, pleasure steamers or boots.

The sandwich composite elements according to the invention may also findapplication in sports articles for use on land, water or air.

Sandwich composite elements with structural elements as core layeraccording to the invention are highly preferred for application as vanesor rotor-blades for wind powered generating units.

The invention is described in greater detail in the following by way ofexample and with reference to the accompanying drawings. These show in:

FIG. 1: an explosive view of a sandwich composite with a structuralelement according to the invention as core layer;

FIG. 2: a cross-section through a sandwich composite with a structuralelement according to the invention as core layer;

FIG. 3: plan view of a first version of structural element according tothe invention;

FIG. 4: plan view of a second version of structural element according tothe invention;

FIG. 5: plan view of a third version of structural element according tothe invention;

FIG. 6: plan view of a fourth version of structural element according tothe invention;

FIG. 7: plan view of a fifth version of structural element according tothe invention;

FIG. 8: plan view of a sixth version of structural element according tothe invention;

FIG. 9: perspective view of a plastic block for manufacturing structuralelements according to the invention as shown in FIG. 5.

FIG. 1 shows a sandwich composite element 1 with a core layer ofstructural element according to the invention, which is in the form of aplastic sheet 10 (see also FIG. 3). The plastic sheet 10 is made up ofblock-shaped body segments 11 which are joined together at theirtouching side faces 17 via longitudinal and transverse weld seams 12, 13that extend over the said side faces 17. The weld seams 12, 13 formthereby (as seen in plan view) a network-like, stiff structure ofstruts. Provided on each side of the core layer on the outer faces 16 ofthe body segments 11 is an outer layer 2, 3. The outer layers 2, 3 maybe in the form of plastic sheets, fibre reinforced sheets (e.g.fibre-glass reinforced duroplastics or thermoplastics) or metal sheets,in particular aluminium sheets.

FIG. 2 shows a cross-sectional view of a sandwich composite element 1 asin FIG. 1. The outer layers 2, 3 are joined to the core layer by meansof adhesive bonding.

FIGS. 3 to 7 show various versions of structural elements according tothe invention in the form of plastic sheets 10, 20, 30, 40, 50, 60 madeup of body segments 11, 21, 31, 41, 51, 61, of different geometricalshapes and arrangement that are arranged lying next to each other.

The plastic sheets 10, 20 shown in FIGS. 3 and 4 contain body segments11, 21 of rectangular cross-section as viewed in plan view. The bodysegments 11 as shown in FIG. 3 are arranged in a plane next to eachother in several rows 14 and columns 15, whereby the weld joints betweenthe body segments 11 form a network-like structure of intersectinglongitudinal and transverse weld seams 12, 13.

The body segments 21 shown in FIG. 4 are arranged in a plane next toeach other in several rows 24 that are offset with respect to eachother, whereby the weld joints form a structure that has the appearanceof a brick wall with uninterrupted transverse weld seams 23 andlongitudinal weld seams 22 that are offset with respect to each other.

The plastic sheets 30, 40 shown in FIGS. 5 and 6 contain body segments31, 41 of quadratic cross-section as viewing the sheet in plan view. Thebody segments 31 shown in FIG. 5 are arranged next to each other in aplane in several rows 34 and columns 35, whereby the weld joins form anetwork-like structure with intersecting longitudinal and transverseweld seams 32, 33.

The body segments 41 shown in FIG. 6 are arranged in a plane next toeach other in offset rows 44, whereby the weld joins form uninterruptedtransverse weld seams 43 and longitudinal weld seams 42 that are offsetwith respect to each other.

FIG. 7 shows a further version of a plastic sheet 50 having—as viewed inplan view—hexagonal i.e. web-shaped body segments 51 which are arrangedon a plane next to each other and are welded together forming weld seams52.

FIG. 8 shows another version of a plastic sheet 60 according to theinvention having—as viewed in plan view—triangular body segments 61which are arranged on a plane next to each other and are welded togetherforming weld seams 62.

FIG. 9 shows a plastic block 5 made up of column-shaped or rod-shapedfoam bodies 7, whereby the foam bodies 7 are manufactured e.g. by meansof extrusion or by dividing a homogeneous block of foam. The individualfoam bodies 7 are, according to the invention, joined together alongtheir long sides 8 by plastic welding, forming longitudinal andtransverse weld seams 32, 33 respectively.

By sawing or thermally cutting the plastic block 5 is divided intoindividual plastic sheets 30 according to the invention as shown in FIG.5, whereby the plastic sheets 30 are made up of body segments 31 thatare arranged next to each other on a plane.

1-20. (canceled)
 21. A structural element, comprising: a block whichcomprises a plurality of body segments of a foamed, thermoplasticmaterial having a stretched polymer structure in one direction, the bodysegments being arranged next to each other and joined together on aplane defining foam sheets running transverse or perpendicular to theone direction of the stretched polymer structure, wherein the structuralelement is a plastic sheet having opposed relatively large area sides,and relatively small area edges, and where the stretched polymerstructure is in a direction perpendicular to the relatively large areasides, wherein the structural element is made wholly of plastic and thebody segments are welded together at abutting side faces forming weldseams, whereby the weld seams form a substantially pore-freeintermediate plastic layer in the form of a network of stiffening strutsdefined by intersecting weld seams, wherein each of the weld seams havea thickness such that a network strut structure is formed by the weldseams which increases the compressive strength of the structural elementwith respect to surface pressure wherein the structural element has aspecific weight less than 130 kg/m³.
 22. The structural element of claim21, wherein the specific weight is between 20 kg/m³ and 130 kg/m³. 23.The structural element of claim 21, wherein the body segments are madeof a closed-cell foam material.
 24. The structural element of claim 21,wherein the thermoplastic material is polyethylene-terephthalate (PET)or styrene/acrylnitrile-copolymer (SAN).
 25. The structural element ofclaim 21, wherein the weld seams comprises melted plastic of the bodysegments.
 26. The structural element of claim 21, wherein the bodysegments are formed from lengths cut from rod-shaped or column-shapedfoam bodies.
 27. The structural element of claim 26, wherein therod-shaped or column-shaped foam bodies are formed by means ofextrusion, and the direction of extrusion in the body segmentsmanufactured from the foam bodies lies substantially parallel to a lineof intersection of two intersecting weld seams.
 28. The structuralelement of claim 21, wherein the body segments have a polygonal shape.29. The structural element of claim 21, wherein the structural element(10) is a plastic sheet.
 30. The structural element of claim 21, whereinthe weld seams form a structure with uninterrupted transverse weld seamsand longitudinal weld seams that are offset with respect to each other.31. The structural element of claim 21, wherein the body segments arearranged on a plane next to each other and are welded together formingthe weld seams.